The present invention relates to apparatuses for monitoring the condition of a tire attached to a vehicle, and, more particularly, to techniques for judging the condition of a battery installed in a transmitter that is attached to a tire.
Conventionally, a wireless, tire condition monitoring apparatus informs a driver of the condition of each tire attached to a vehicle through an indicator located in a passenger compartment. A transmitter is attached to each tire. The transmitter measures the pressure and temperature in the associated tire and wirelessly transmits data representing the measurements. A receiver is located in the vehicle's body frame and receives the data from the transmitter. The receiver obtains necessary information from the data and instructs the indicator to indicate the information as needed.
Since each transmitter is powered by a battery, the transmitter stops operating if the associated battery's life ends. Thus, when the battery's life is ending, the driver needs to be informed of the condition of the battery through the indicator, which is located in the passenger compartment. Accordingly, the conventional monitoring apparatus transmits data representing the voltage of each battery, together with the data representing the condition of each tire. If the voltage of the battery is equal to or smaller than a predetermined reference value, the receiver judges that the life of the battery is ending. The receiver then instructs, for example, the indicator to indicate the judgment.
However, the battery voltage is affected by the ambient temperature to which the battery is exposed. That is, the lower the ambient temperature is, the lower the battery voltage is. Particularly, the temperature variation in the tire in which the battery is located is relatively large. Thus, the condition of the battery cannot be accurately judged if the judgment is based on only the comparison between the battery voltage and the reference value. In other words, if the judgment is based only on such a comparison, the condition of the battery may be erroneously judged.